The invention relates to a method for offset compensation of a magnetoresistive position or angular position measuring system, which comprises at least one Wheatstone bridge with four locally offset magnetoresistive resistors to which a DC voltage is applied and past which a permanent magnet is moved, wherein the respective bridge voltage is measured.
It is currently common to use magnetoresistive sensors for position and angular position measurement. These consist of magnetoresistive resistors which are arranged in the manner of a Wheatstone bridge on a generally disk-shaped support in front of which a permanent magnet rotates. To obtain a linear output signal, two bridges are used whose resistors are arranged at a mechanical angle to one another of 45xc2x0, such that one bridge generates a sinusoidal signal and the other a cosinusoidal signal. The ARCTAN function then allows a linear function over 180xc2x0 as the output signal. The signals are evaluated, for example, in a mixed-signal IC which together with the sensor constitutes a measuring system.
Due to the production process, the magnetoresistive resistors are non-identical, which, however, may be compensated in the IC when the measuring system is started up. This compensation thus takes place individually for each individual measuring system. During a final measuring process, the sensor is magnetically excited in such a way prior to delivery that the individual offset, i.e. the respective offsets of the two bridges, may be determined from the signals. These values are written into an EEPROM in the IC and subsequently taken into account by the IC.
EP 1 001 248 A2 describes a method for one-off calibration on the production line of a measuring system with magnetoresistive sensors, by exposing the bridges to external magnetic fields of different directions. The output voltage supplied by a Wheatstone bridge after removal of a direct magnetic field is thus a measure for the respective offset voltage, which is used for calibration.
In addition to the tolerances caused by the production process, the magnetic resistors are also subject to aging, which causes further offset errors. If it is to be dependable with regard to new offset errors, therefore, the system would have to be recalibrated from time to time under factory-like conditions, which entails a level of expenditure which the user is naturally reluctant to contemplate. Instead, they would rather put up with an error.
It is therefore an object of the invention to provide a method by which a subsequent offset compensation of magnetoresistive position or angular position measuring systems is also possible on site.
According thereto, when a measuring system is started up, a calibration cycle is performed by
moving the permanent magnet past the bridge(s) for a measuring cycle,
evaluating the extreme values of the bridge voltages measured during this procedure by deriving the mean values thereof, and
deriving from these mean values corresponding compensation values in the form of offset values to be taken into consideration by the measuring system, said compensation values being stored in a register.
Such a calibration cycle may be performed whenever the measuring system is switched on. However, provision may alternatively be made for a calibration cycle to be performed only after an appropriate number of measuring cycles, for example every tenth cycle. For certain applications, for example for motor vehicles, provision may also be made for only authorized personnel to be permitted to perform calibration, such that it could then be performed during scheduled maintenance.
In the latter case, the EEPROM into which the compensation values were written during factory calibration could be used as the register. In the event of a basic calibration cycle being performed each time the measuring system is started up, a volatile memory may alternatively suffice as the register.
The invention has the advantage that offset calibration may be performed automatically or as desired throughout the entire service life of the measuring system. The calibration cycle may be incorporated into the start-up procedure of a machine, such that it runs automatically after the machine is switched on, by performing a calibration of an angular position measuring system in a no-load state, for example. For this, a half revolution of the permanent magnet is sufficient. A measuring time of less than 100 ms is sufficient for the calibration cycle.
Provided that a high field is used in the measuring system, i.e. that the sensors reach saturation with regard to amplitude and thus only the phase angle but not the sensitivity of the measuring signal is of relevance, it is ensured that the measured quantity is run through fully in the event of a half-revolution of the permanent magnet and thus the measured extreme values are guaranteed xe2x80x9ctruexe2x80x9d values. This means that the determined offset values are always xe2x80x9ctruexe2x80x9d.